1. Field of the Invention
The invention relates generally to an automatic transmission and more particularly to an oil pressure clutch mechanism of the automatic transmission.
2. Description of Prior Art
The automatic transmission which has plural oil pressure clutches has a problem in that oil leaks out from a first passage (i.e., a passage for one oil pressure cylinder which controls the frictional engagement of one oil pressure clutch) to a second passage (i.e. a passage for another oil pressure cylinder which controls the frictional engagement of the other oil pressure clutch). Therefore, when the other oil pressure clutch is in disengagement, oil leaks out from the first passage to the second passage because of the differential pressure and flows into the second oil pressure cylinder, and the leaked oil generates fluid pressure in response to being subjected to centrifugal force created by the rotation of the parts of the automatic transmission. The centrifugally generated oil pressure acts against the piston of the other oil pressure cylinder to engage the frictional engaging parts of the other oil pressure clutch. In the frictional engaging parts of the other oil pressure clutch, therefore, the frictional material burns out, the output shaft locks out and the parts are damaged.
In order to prevent the above-mentioned oil leakage and the clutch engagement through the centrifugally generated oil pressure, the patent application TOKU-KAI-SHO 62-4932 suggests an oil pressure clutch mechanism. The oil pressure clutch comprises a pair of oil pressure chambers which are formed on both axial ends of a piston housing which is fixed to a clutch shaft. A pair of clutch pistons are inserted in the oil pressure chambers so as to move axially therein. An oil circuit supplies oil to the oil pressure chambers to move the clutch pistons. A hole is formed in the piston housing for communicating the pair of oil pressure chambers. A drain port communicates the hole and a region external to the piston housing. A spool is slidably interposed in the hole, which has (i) a flange of larger diameter than the end of the hole which projects in one oil pressure chamber, (ii) a plate of larger diameter than the other end of the hole which projects communication with the other oil pressure chamber, and (iii) a recess along the length of the hole. A spring is disposed between the flange and the side end of the clutch piston.
This conventional oil pressure clutch arrangement for axially side-by-side clutches prevents clutch engagement by centrifugally generated pressure of leaked oil. This conventional clutch arrangement has certain shortcomings, however, namely cost because of increased parts, difficulty in adapting to clutches which are coaxially arranged (as opposed to a side-by-side arrangement), and the impossibility of setting both of the clutches at the same time in a state of engagement or disengagement.